The present invention relates to a cellulosic microporous filtration membrane and to an environmentally friendly method for producing the membrane. Symmetrical cellulosic filtration membranes are known. See DE 44 38 381 A1. Asymmetric membranes are known as well, which are typically made from synthetic polymers such as polysulfone. See EP 1 118 377 A2. Asymmetric membranes are characterized by a pore size gradient throughout the thickness of the membrane, with the smallest diameter pores on the active filtration layer which is situated on the outside of the membrane (feed side or gas side), while the pores with the largest diameters are in an underlying porous layer. With asymmetric membranes, the pore size gradient generally gradually increases throughout the thickness of the membrane. Symmetrical membranes display a substantially uniform pore size diameter throughout the entire thickness of the membrane. A region which has uniformly sized pores is referred to as an isotropic region.
U.S. Patent Application Publication 2003/0038081 A1 describes asymmetric cellulose membranes having first and second layers, as well as an intermediate porous layer between the first and second porous layers. The same Publication describes a membrane provided with a so-called “funnel with a neck” structure and that has an asymmetric and an isotropic region, with the pores in the isotropic region having a uniform size.
EP 0 723 993 B1 describes a method for the production of a cellulose acetate film from a cellulose acetate solution using a solvent consisting of an ester having 3 to 12 carbon atoms.
DE 199 25 929 C2 describes a microfiltration membrane based on cellulose acetate containing hydrophilic silicic acid, and a conventional method for the production of the membrane.
The primary object of the present invention is to provide a cellulosic filtration membrane that has excellent filtration performance and service life, as well as to provide a method of making such a membrane that is particularly environmentally friendly.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings.